Custom Field - Plans (as an option)

Hi Tuan.

With current implementation of custom field Checkboxes it's impossible to change options (and amounts respectively) during active subscription period without canceling subscription and resubscribing again with different options chosen.

We have few main products (plans) and multiple additional plans to main products like projections and teams. Right now users have ability to change projections and number of team independently from main plan by subscribing/cancelling those additional plans. But they have to subscribe to each additional plan individually. We would like to give users ability to subscribe to additional projections and teams during initial subscription to the main plan. So basically we need ability to subscribe to multiple plans at once and that would be possible with new custom field type.

It would be great to be able to set some plans checkboxes as checked by default.
Small remark: each plan should be processed as a separate transaction.
Never mind about Add a team spinner on the screen shot below, as it can be done thru override and styling.

Thanks!